Cutting tools used for machining work pieces of metal, such as steel, aluminum, titanium, etc., typically include a tool holder for mounting removable cutting inserts. During operation of the cutting tool, a centrifugal force Fz is generated. In high speed milling operations, centrifugal force Fz becomes significant due to its relationship with spindle speed (growing in power 2 proportional to spindle speed) in accordance with the equation Fz=M*W2*R, where M is the mass of the insert, W is the speed of rotation, and R is distance between the rotational center of the tool and the position of the insert's center of mass. The cutting tool system becomes unbalanced when the centrifugal force Fz exceeds a clamping force.
With reference to FIG. 20, the relationship of cutting speed on cutting force is shown. As illustrated in FIG. 20, an increase in cutting speed typically supports a reduction of the cutting force. In particular, the hardness and strength of the workpiece material is decreased due to increased temperature in the cutting zone at high speeds. At the same time, centrifugal force applied to the insert Fz is growing dramatically (square to RPM). There is a point where the centrifugal force becomes larger than the amplitude of the cutting force. For high speed machining, the centrifugal force will be typically larger than cutting force. High speed machining typically has RPMs in excess of 10,000. However, most traditional designs of inserts and clamping mechanisms are designed for low centrifugal force and may fail during high speed machining.
At high cutting speeds, centrifugal forces applied to the cutting element significantly exceed cutting forces. If the clamping system fails, kinetic energy accumulated during rotation will be released in accordance with EK=(½)MV2 wherein M is the mass of the object and V is the linear velocity at the moment of failure. Because of the above-noted relationship, one of most important factors and requirement at high speeds is safety of the rotating tool.
Cutting tools have been designed with an open pocket for location of a cutting insert. A disadvantage with this particular design is that high centrifugal force is applied to the clamping screw, which is already pre-stressed during clamping. When the screw fails, the insert is “free to fly” to release significant kinetic energy.
There are known designs of milling cutters which utilize inserts with serrations on a bottom surface or surface opposed to the cutting plane of the insert, as shown, for example, in U.S. Pat. Nos. 5,924,826, 5,810,518, and 6,921,234. A disadvantage of these systems is vulnerability of the cutter body for collisions and breakage of the inserts. Because serrations are provided on the insert bottom to support insert load, and being extended to the maximum, damage occurred is usually very severe and repair is very difficult. Another disadvantage of these designs is the absence of axial adjustment mechanism, which would allow precision positioning of the inserts without costly grinding in assembly.
Finally, as discussed above, a single screw is used for securing the insert in the open pocket. If the screw fails, the entire insert will be released from the pocket. There is no additional features or redundancy that further prevents the insert from being released during operation should the screw fail. Therefore, there is a need in the art for a cutting tool where the clamping screw is not subjected to the cutting load, and does not take significant centrifugal load. In addition, there is a need in the art for redundant securing features in high speed applications.
While the serrations provided on the bottom surface can absorb a significant portion of centrifugal force, they can not absorb 100% of the centrifugal force. Due to the angular nature of the serration's cross section, there still is a portion of centrifugal force, which has “lifting” power. When cutter rotation speed is increasing, a portion of centrifugal force reflected through serrations to the clamping screw is growing also. At some point, the centrifugal force will cause the insert to lift and make the cutting tool fail. Another disadvantage of this type of cutting tool is that if the insert uses two cutting edges, the second edge will experience premature wear by hitting chips generated during the cut. Also, ongoing chips contact elements of any adjustment mechanism, which further creates premature wear.
For cutting tools having a carbide tip, the ability to stay retained in the insert becomes even more crucial. Because carbide is twice as dense as steel, it will be subjected to a larger centrifugal load. Due to its weight, it is more likely to fly out of the pocket. In addition, the carbide tip takes a majority of the cutting load, and could become separated from the body. For applications using carbide tips, it is important that should a failure occur, that only a small portion of the carbide tip be released, causing a smaller likelihood of damage.
Accordingly, there is a need in the art for an improved milling cutter for high speed milling operations. In addition, there is a need in the art for an insert that is more resistant to wear and failure. Finally there is a need in the art for redundant securing features in high speed applications, particularly those using heavier materials such as carbide.